1. Technical Field
The present invention relates to cascaded distortion compensation for analog optical systems and, more particularly, to an arrangement using a series connection of a plurality of separate pre- and/or post-distortion compensation components, each component capable of providing specific functionality.
2. Description of the Prior Art
A growing area for analog optical communication systems is the common antenna television (CATV) network. In particular, recent advances in long wavelength distributed feedback (DFB) laser technology have made possible the transport of multiple CATV channels over one single mode fiber at .lambda.=1.3 .mu.m. See, for example, "Lightwave subcarrier CATV transmissions systems", by T. E. Darcie et al. appearing in IEEE Trans. Microwave Theory Tech., Vol. MTT-38, p. 524, 1990. The low levels of analog distortions and noise from the DFB lasers have been found to satisfy the system requirements such that the presence of many channels over a common communication path does not noticeably affect the reception of any particular channel.
It has been well documented, however, that nonlinearities of the DFB laser affect the composite second order (CSO) distortion of the system. Pre-distortion circuits have been developed to compensate for the laser nonlinearity, one exemplary arrangement being disclosed in U.S. Pat. No. 4,992,754 issued to H. A. Blauvelt et al. In this particular arrangement, the distortion is compensated by applying a pre-distorted signal equal in magnitude and opposite in sign to the distortion introduced by the DFB laser. The input signal is split into two paths with the primary part of the signal applied directly to the device, including a time delay to compensate for delays in the secondary path. A pre-distorter in the secondary path generates harmonic signals, the amplitude of which are adjusted to match the amplitude of the distortion. A tilt adjustment is made to compensate the amplitude of the pre-distortion for the frequency dependence of distortion. A fine adjust of the delay is also included so that the phase of the predistortion signal is properly related to the phase of the primary signal.
Additional sources of nonlinearities not discussed in the Blauvelt et al. reference, for example, the interaction of FM chirp intrinsic to a DFB laser with fiber dispersion, can also affect the system performance, as discussed in the article "Dispersion-Induced Composite Second-Order Distortion at 1.5 .mu.m", by E. E. Bergmann et al. appearing in IEEE Photonics Tech. Lett., Vol. 3, No. 1, January 1991, at p. 59. As discussed in the Bergmann et al. reference, dispersion nonlinearity can be counteracted by utilizing dispersion-shifted fiber, reducing laser chirp, or limiting applications to relatively short spans (e.g., &lt;3 km). Exemplary predistortion compensation for this combination is discussed in an article entitled "Electrical predistortion to Compensate for Combined Effect of Laser Chirp and Fibre Dispersion", by H. Gysel et al. appearing in Electronic Letters, Vol. 27, No. 5 at pp. 421-3. Gysel et al. discusses the utilization of a varactor diode/inductor combination to "build in" the inverse of the expected distortion in the signal as applied to the optical transmitting device.
Recently, doped fiber amplifiers have become available which can be used in a CATV network to significantly increase the link loss budget. In particular, the erbium doped fiber amplifier (EDFA) is an attractive component since it exhibits high saturated output power, polarization independent amplification, and low intrinsic optical noise. The high saturated output power of an EDFA is of particular importance to CATV transport and distribution applications. Furthermore, its saturated gain characteristic does not respond to input signal variations at speeds faster than a few kilohertz because of the small absorption and stimulated emission cross sections, as well as the long metastable lifetime of the erbium ions. However, when an EDFA is used to amplify an analog optical AM CATV multiple carrier signal from a directly modulated DFB laser, an increase in the system distortion is observed.
In general, the combination of the above-noted dispersion sources, along with other nonlinear components contained within the communication system, such as external modulators and/or erbium-doped fiber amplifiers, results in an overall system-based nonlinear effect which may distort the system performance. Prior art compensation techniques, which address distortion at the component level (i.e., prior to installation in a communication system), cannot provide adequate compensation for the overall analog communication system.
Thus, a need exists for reducing the signal distortion attributed to the system-level nonlinearity present within an operating analog optical communication system.